Copied from Debian.

--- celestia-1.6.0+dfsg.orig/src/celengine/frametree.h
+++ celestia-1.6.0+dfsg/src/celengine/frametree.h
@@ -49,7 +49,7 @@ public:
 
     bool isRoot() const
     {
-        return bodyParent == NULL;
+        return bodyParent == 0;
     }
 
     bool updateRequired() const
